Can stock with differential protective coatings

ABSTRACT

DIFFERENTIALLY TREATED DIFFERENTIAL TINPLATE HAVING A TRIVALENT CHROMIUM OXIDE COATING THEREON TO THE EXTENT THAT ABOUT 1.00 TO 1.50 MG. OF CHROMIUM PER SQUARE FOOT IS PROVIDED ON THE HEAVILY TINNED SIDE AND ABOUT 0.40 TO 0.80 MG. CHROMIUM PER SQUARE FOOT IS PROVIDED ON THE REVERSE LIGHTLY TINNED SIDE. THE PROCESS INVOLVES THE CATHODIC ELECTROLYSIS OF THE DIFFERENTIAL TINPLATE IN A DILUTE SODIUM-DICHROMATE SOLUTION HAVING A PH OF FROM 3.5 TO 5.5 AT A TEMPERATURE WITHIN THE RANGE OF 160 TO 200*F. THE HEAVILY TINNED SIDE THEREOF IS SUBJECTED TO AN ELECTRICAL CURRENT OF 100 TO 170 COULOMBS PER SQUARE FOOT WHILE THE REVERSE SIDE IS SUBJECTED TO NOT MORE THAN 45 COULOMBS PER SQUARE FOOT, AT CURRENT DENSITIES WITHIN THE RANGE 10 TO 170 AMPERES PER SQUARE FOOT.

United States Patent O 3,749,651 CAN STOCK WITH DIFFERENTIAL PROTECTIVE COATINGS James E. Bird, Murrysville, and Paul R. Carter, Monroeville, Pa., assignors to United States teel Corporation No Drawing. Continuation-impart of abandoned application Ser. No. 840,519, July 9, 1969. This application Aug. 27, 1971, Ser. No. 175,728

Int. (ll. C2311 11/00 US. Cl. 204-56 R 2 Claims ABSTRACT OF THE DISCLOSURE Differentially treated differential tinplate having a trivalent chromium oxide coating thereon to the extent that about 1.00 to 1.50 mg. of chromium per square foot is provided on the heavily tinned side and about 0.40 to 0.80 mg. chromium per square foot is provided on the reverse lightly tinned side. The process involves the cathodic electrolysis of the differential tinplate in a dilute sodium-dichromate solution having a pH of from 3.5 to 5.5 at a temperature within the range 160 to 200 F. The heavily tinned side thereof is subjected to an electrical current of 100 to 170 coulombs per square foot while the reverse side is subjected to not more than 45 coulombs per square foot, at current densities within the range to 170 amperes per square foot.

This application is a continuation-in-part of application Ser. No. 840,519, filed July 9, 1969, now abandoned.

BACKROUND OF THE INVENTION There are many known processes for treating tinplate to further enhance its corrosion resistance characteristics. Of particular concern are processes which inhibit corrosion of particularly corrosive foods, such as sulfur bearing food products which cause sulfide staining, or other corrosive foods which may cause detinning.

US. patent application Ser. No. 14,283, filed Feb. 26, 1970, discloses a process for treating tinplate which provides exceptional resistance to oxidation, sulfide staining, detinning and abrasion: The process comprises an electrolytic treatment whereby the tinplate strip is made cathodic in a dilute sodium-dichromate solution thereby depositing a trivalent chromium oxide, such as chromic sesquioxide, Cr O -xH O, which provides a suitable protective coating normally providing from 1.0 to 1.5 mg. chromium per square foot. The coating provides exceptional resistance to sulfide staining and detinning, and further provides good adherence properties for lacquer, and resistance to oxide growth during storage. The crux of that inventive process resides in the application of a large quantity of electricity at reasonably high temperatures, i.e., 100 to 170 coulombs per square foot at 160 to 200 F.

In practicing the above inventive process, we have learned that the protective chromium oxide coating does indeed provide the alleged beneficial results when applied to conventional tinplate grades sold commercially, i.e., those having tin coatings of at least about 0.50 pound of tin per base box. This is equal to an individual tin surface of about 0.00115 pound of tin per square foot. When such a chromium oxide coating is applied over tin coatings of less than 0.00115 pound of tin per square foot (i.e., less than 0.50 pound per base box), the solderability thereof is adversely affected. In fact, the chromium oxide coating so adversely affects the solderability thereof that the such can stock cannot be satisfactorily soldered by the jet soldering machines used by the can making industry.

Although corrosion resistant tinplate is seldom sold with coatings lighter than 0.00115 pound of tin per square foot of surface area, i.e., less than 0.50 pound per base box, such coatngs are quite common on one surface of differential tinplate. Differential tinplate as used herein, refers to tinplate stock having a heavier tin coating on one side thereof as compared to the other. For economic reasons, differential tinplate is commonly used by the can making industry to provide a can with a heavy tin coating on the inside, and a lighter tin coating on the outside where corrosive environments are less severe. Differential tinplate is sold with various combinations of diiferent tin coatings thereon. The most common however are those having a lightly tinned side of 0.000575 pound of tin per square foot of surface area, i.e. a coating comparable to 0.25 pound per base box.

It follows, therefore, that although the process of patent application Ser. No. 14,283 is perfectly suitable for conventional tinplate, it is not suitable for differential tinplate having tin coatings on the lightly tinned side of less than 0.00115 pound of tin per square foot of surface. When the process is applied to such differential tinplate, the solderability of the lightly tinned side is so adversely affected that can manufacturers cannot produce an acceptable solder thereon.

SUMMARY OF THE INVENTION This invention is predicated upon our discovery that differential tinplate, namely those grades having a lightly tinned side of less than 0.00115 pound of tin per square foot of surface, can be suitably coated with chromium oxide to enhance its resistance to oxidation, sulfide staining and detinning and at the same time maintain good solderability of the lightly tinned side if a differential chromium oxide coating is provided. The chromium oxide coating on the heavily tinned side should of course substantially be that provided by the process taught in patent application Ser. No. 14,283, i.e., normally from 1.0 to 1.5 mg. chromium per square foot of surface. The lightly tinned side, however, should have a chromium oxide coating no greater than sufficient to provide from 0.4 to 0.8 mg. chromium per square foot of surface, which can be assured by providing a quantity of electricity of no more than about 45 coulombs per square foot of surface.

It is therefore an object of this invention to provide a differentially treated differential tinplate having exceptional resistance to sulfide staining and detinning and at the same time maintain good solderability.

It is another object of this invention to provide a process for treating differential tinplate to provide a heavy chromium oxide coating on the heavily tinned side of the tinplate, thereby enhancing resistance to oxidation, sulfide staining and detinning, and to provide a lighter chromium oxide coating on the reverse side to maintain good solderability of the tinplate.

DESCRIPTION OF THE PREFERRED- EMBODIMENT Although this invention could be applied to any dif ferential tinplate, it is mostly beneficial to those grades wherein the heavily tinned side exceeds 0.00115 pound of tin per square foot of surface, i.e., 0.50 pound per base box, and the lightly tinned side contains less than that amount down to about 0.000575 pound per square foot of surface. Concerning commercial grades therefore, this invention is most advantageously applied to the commercial grades 50-25, -25, 25 and -25. (The coating weight 135 would have a tin coating of 0.00311 pound of tin per square foot of surface.) Even Without chromium oxide coatings, these grades of differential tining as described in patent application Ser. No. 14,283, the solderability is further adversely affected to the point where can makers can no longer 'get acceptable soldered joints.

The crux of this invention resides in our discovery that the solderability of the lighter tin coated side of differential tinplate is not adversely affected, even when baked, if a like trivalent chromium oxide coating is electrolytically deposited thereon providing only from 0.40 to 0.80 mg. chromium per square foot of surface. Since the heavier tin coated side normally had good solderability not affected by the amount of chromium oxide coating, this side may then be given a more extensive protective coating, e.g., normally in excess of 1 mg. chromium per square foot to provide suitable resistance to oxidation, sulfide staining and detinning. For the heavily tinned side therefore, the process as taught in patent application Ser. No. 14,283 is applied. That is to say, the tinplate is immersed into a dilute sodium-dichromate solution having a pH of from 3.5 to 5.5 at a temperature within the range 160 to 200 F. The tinplate is made cathode adjacent to a conductive anode on the heavily tinned side of the tinplate, and sufficient electrical current is applied to effect from 100 to 170 coulombs per square foot of tinplate surface. The applied current density does not appear to be critical since any current density within the range normally obtainable with commercial equipment will be effective. Specifically we have produced the desired chromium oxide coating at various current densities between 10 and 170 amperes per square foot of surface.

To assure that only 0.4 to 0.8 mg. chromium per square foot is applied to the lightly tinned side of the tinplate, the total quantity of electrical current applied to that side directly should be no greater than 45 coulombs per square foot. Since it is preferred that both sides be treated simultaneously, the treating time for both sides will be the same. Therefore, it is necessary to merely utilize a correspondingly lower current density on the lightly tinned side of the tinplate sufficient to effect no more than 45 coulombs per square foot. It will be noted that there is no minimum value given for the coulombs applied to the lightly tinned side of the tinplate. Hence, any value between zero and 45 coulombs per square foot is sufficient. Indeed, the desired product can be made without any current directly applied to the lightly tinned side of the tinplate. This is because the current applied to the heavily tinned side is so heavy at 100 to 170 coulombs per square foot that there is suflicient wraparound current to deposit the minimum 0.4 mg. chromium per square foot on the lightly tinned side, even without a current directly applied thereto. The current density for the lightly tinned side of the tinplate may therefore be varied from to 200 amperes per square foot.

As a specific example of the process of this invention, we have passed 100-25 differential tinplate strip through a sodium-dichromate solution containing 20 to 30 grams of the salt per liter, while maintaining the solution at a pH of about 4.0 and a temperature of about 190 F. During the passage, the strip was connected as cathode in an electrolyzing circuit to a suitable current source. Conductive anodes of mild steel were disposed about 1 /2 inches from each face of the strip. The heavily tinned side of the strip was subjected to an electrical current of 150 coulombs per square foot while the lighter tinned side was subjected to 40 coulombs per square foot. The strip was then rinsed in water and given the usual finishing treatment, i.e., drying, oiling and coiling. The differential applied current was effected by providing a larger anode surface area for the heavy current side. Specifically, the strip was advanced through two separate tanks. In the first tank, the strip was passed adjacent to two 5-foot anodes, both on the heavily tinned side of the strip. In the second tank, three anodes were used, two again on the heavily tinned side of the strip, and one on the lightly tinned side. The resulting sheet had 1.25 mg. chromium per square foot on the heavier tinned side and 0.80 mg. chromium per square foot on the other side. Can stock cut from the strip was enameled and baked in accordance with usual canning procedures and easily soldered with jet soldering units.

Although the above described specific example did utilize the optimum process parameters, these parameters may be varied somewhat depending upon personal preferences and economics. Specifically, the salt concentration of the sodium-dichromate solution may be varied between 20 and 30 grams per liter, while the pH value may vary between 3.5 and 5.5 but is preferably maintained at 4.0 to 4.2. The temperature of the solution should be within the range 160 to 200 F. or ideally at from 180 to 190 F. In order to provide a good heavy protective coating on the heavily coated side which will be resistant to sulfide staining and detinning, the total current applied to that side should be within the broad range of to 170 coulombs per square foot, or preferably to coulombs per square foot, yielding from 1.00 to 1.50 mg. of chromium per square foot. In order to enhance solderability of the lightly tinned side, however, the total current applied to that side should be maintained at no more than 45 coulombs per square foot, yielding 0.40 to 0.80 mg. of chromium per square foot.

We claim:

1. A method of treating differential tinplate to maintain good solderability and resistance to oxidation, sulfide staining and detinning comprising electrolyzing the differential tinplate as cathode in a solution consisting essentially of a sodium-dichromate solution of from 20 to 30 grams per liter, having a pH of from 3.5 to 5.5 and at a temperature between and 200 F supplying a quantity of electrical current to the heavily tinned side of said differential tinplate within the range 100 to coulombs per square foot at current densities of from 10 to 170 amperes per square foot, and supplying a quantity of electrical current to the lightly tinned side or no more than 45 coulombs per square foot at current densities no greater than 200 amperes per square foot.

2. A method according to claim 1 in which said pH is maintained at a value of 4.0 to 4.2, said temperature is held within the range 180 to F., and the quantity of electrical current supplied to said heavily tinned side of said differential tinplate is from 125 to 150 coulombs per square foot.

References Cited UNITED STATES PATENTS 3,491,001 1/ 1970 Shackleford et al. 204-56 R FOREIGN PATENTS 863,234 3/1961 Great Britain 204--56 R FREDERICK C. EDMUNDSON, Primary Examiner 

